A metal-air cell is a type of battery that utilizes the same energy storage principles as a more conventional cell such as a lithium ion, nickel metal hydride, nickel cadmium, or other cell type. Unlike such conventional cells, however, a metal-air cell utilizes oxygen as one of the electrodes, typically passing the oxygen through a porous metal electrode. The exact nature of the reaction that occurs in a metal-air battery depends upon the metal used in the anode and the composition of the electrolyte. Exemplary metals used in the construction of the anode include zinc, aluminum, magnesium, iron, lithium and vanadium. The cathode in such cells is typically fabricated from a porous structure with the necessary catalytic properties for the oxygen reaction. A suitable electrolyte, such as potassium hydroxide in the case of a zinc-air battery, provides the necessary ionic conductivity between the electrodes while a separator prevents short circuits between the battery electrodes.
Due to the use of oxygen as one of the reactants, metal-air cells have some rather unique properties. For example, since the oxygen does not need to be packaged within the cell, a metal-air cell typically exhibits a much higher capacity-to-volume, or capacity-to-weight, ratio than other cell types making them an ideal candidate for weight sensitive applications or those requiring high energy densities.
While metal-air cells offer a number of advantages over a conventional rechargeable battery, most notably their extremely high energy density, such cells also have a number of drawbacks. For example, care must be taken to avoid the undesired evaporation of electrolyte, especially in high temperature, low humidity environments. It is also necessary to ensure that there is a sufficient supply of air to the cells during discharge cycles, and means for handling the oxygen emitted from the cells during the charge cycles. Another potential disadvantage of a metal-air cell is the power available on discharge. Due to the kinetics of the electrode reactions, the maximum discharge rate is far lower than that of many other types of cells, such as lithium-ion cells.
Accordingly, while metal-air cells offer some intriguing benefits, such as their high energy densities, their shortcomings must be taken into account in order to successfully integrate the cells into a system. The present invention provides such a system by combining a metal-air battery pack with a conventional battery pack in order to gain the benefits associated with each battery type.